A network device, such as switching hub, is used in a local area network to interconnect a plurality of nodes. Since the network device is not operated daily, the network device is located at a less visible place such as between desks in an office or under a floor of the office. When the network configuration is changed or a countermeasure at a time of trouble is carried out, it is necessary to specify a target network device on site, but it is not easy to find the target network device located at such place.
Further, a connection in a local area network may be made erroneously. For example, both ends of a cable are connected to a same network device, or a connection is made to flow data circularly. It may cause a loop in which a broadcast frame or a frame having an unknown address permanently circulates in the network, which may cause the network down. In order to clear the loop, a network administrator specifies a network device causing the loop, and then visits the site and clears an incorrect connection of the network device. As an example of a method for specifying the network device causing the loop, the network administrator may send a predetermined command from an administration computer and specifies an IP address of the network device causing the loop. Also, JP-A-2007-124184 describes a method and device for detecting the loop. However, as described above, since it is difficult to specify a network device on site, it takes much time to specify the network device. Therefore, network trouble due to the loop cannot be quickly restored.